Is the thermal resistance coefficient of high-power LEDs constant?

In this paper, we discuss the variations of thermal resistance coefficient from junction to board (Rθjb) for high-power light-emitting diodes (LEDs) as a result of changes in power dissipation and ambient temperature. Three-watt white and blue LED packages from the same manufacturer were tested for Rθjb at different input power levels and ambient temperatures. Experimental results show that Rθjb increases with increased input power for both LED packages, which can be attributed to current crowding mostly and some to the conductivity changes of GaN and TIM materials caused by heat rise. With increasing ambient temperature, Rθjb increased but not as much as what was observed with drive current increase.